An aqueous solution in contact with an untreated metal substrate results in corrosion of the untreated metal substrate. Therefore, a metal article, such as a metal container for a water-based product, like a food or beverage, is rendered corrosion resistant in order to retard or eliminate interactions between the water-based product and the metal article. Generally, corrosion resistance is imparted to the metal article, or to a metal substrate in general, by passivating the metal substrate, or by coating the metal substrate with a corrosion-inhibiting coating.
Investigators continually seek improved coating compositions that: (a) have good barrier properties to reduce or eliminate corrosion of a metal article, and (b) do not adversely affect an aqueous product packaged in the metal article. For example, investigators have sought to improve the imperviousness of the coating in order to prevent corrosion-causing ions, oxygen molecules, and water molecules from contacting and interacting with a metal substrate. Imperviousness can be improved by providing a thicker, more flexible, and more adhesive coating, but often, improving one particular advantageous property is achieved at the expense of another advantageous property.
In addition, practical considerations limit the thickness, adhesive properties, and flexibility of a coating applied to a metal substrate. For example, thick coatings are expensive, require a longer cure time, can be esthetically unpleasing, and can adversely affect the process of stamping and molding the coated metal substrate into a useful metal article. Similarly, the coating should be sufficiently flexible such that the continuity of the coating is not destroyed during stamping and molding of the metal substrate into the desired shape of the metal article.
Investigators also have sought coatings that possess chemical resistance in addition to corrosion inhibition. A useful coating for the interior of a metal container must withstand the solvating properties of a product packaged in the metal container. If the coating does not possess sufficient chemical resistance, components of the coating can be extracted into the packaged product and adversely affect the product. Even small amounts of extracted coating components can adversely affect sensitive products, like beer, by imparting an off-taste to the product.
Conventionally, organic solvent-based coating compositions were used to provide cured coatings having excellent chemical resistance. Such solvent-based compositions include ingredients that are inherently water insoluble, and thereby effectively resist the solvating properties of water-based products packaged in the metal container. However, because of environmental and toxicological concerns, and in order to comply with increasingly strict governmental regulations, an increasing number of coating compositions are water based. The water-based coating compositions include ingredients that are water soluble or water dispersible, and, therefore, cured coatings resulting from water-based coating compositions can be more susceptible to the solvating properties of water.
Epoxy-based-coatings and polyvinyl chloride-based coatings have been used to coat the interior of metal containers for foods and beverages because these coatings exhibit an acceptable combination of adhesion to a metal substrate, flexibility, chemical resistance, and corrosion inhibition. However, epoxy-based coatings and polyvinyl chloride-based coatings have serious disadvantages that investigators have not overcome.
Coatings based on polyvinyl chloride or related halide-containing vinyl polymers, like polyvinylidene chloride, possess the above-listed advantageous properties of chemical resistance and corrosion inhibition, and are economical. However, curing a polyvinyl chloride or related halide-containing vinyl polymer can generate toxic monomers, such as vinyl chloride, a known carcinogen. In addition, the disposal of a halide-containing vinyl polymer, such as by incineration, also can generate toxic monomers. The generated vinyl chloride thereby poses a potential danger to workers in metal can manufacturing plants, in food processing and packaging plants, and at disposal sites. Disposal of polyvinyl chloride and related polymers also can produce carcinogenic dioxins and environmentally harmful hydrochloric acid.
Government regulators, therefore, are acting to eliminate the use of polyvinyl chloride-based coating compositions that contact food, and thereby eliminate the environmental and health concerns associated with halide-containing vinyl polymers. Presently, however, polyvinyl chloride-based compositions are still high-volume coatings used to coat the interior of food and beverage containers.
To overcome these environmental concerns, epoxy-based coating compositions have been used to coat the interior of food and beverage containers. However, epoxy-based coatings also possess disadvantages. For example, epoxy-based coating compositions are more expensive than polyvinyl chloride-based coating compositions.
A greater disadvantage of epoxy-based coating compositions is the presence of relatively high amounts of a crosslinking agent, such as a phenolic resin or an aminoplast in the composition. Phenolic resins and aminoplasts typically are condensates of formaldehyde with a phenol, a urea, a melamine, or benzoguanamine. Both types of crosslinking agents include free residual formaldehyde or generate free formaldehyde during cure of the epoxy-based composition. The amount of free formaldehyde often is sufficient to adversely affect the taste of various products packaged in a container coated with a cured epoxy-based composition. For example, a product such as beer is very sensitive to low concentrations of free formaldehyde, and taste of the beer is adversely affected. In addition, free formaldehyde poses a health hazard in the work place. Presently, strict exposure limits are set to protect workers from exposure to formaldehyde.
Various patents and publications disclose waterborne coating compositions for metal cans. In general, prior patents disclose coating compositions including waterborne thermoset resins for use as can coatings. The thermoset resins are formulated with a crosslinking agent to provide crosslinked films during cure, as demonstrated by the resistance of the cured coating to the effects of organic solvents such as methyl ethyl ketone. These waterborne resins include significant amounts of organic solvents, i.e., have a high VOC. The organic solvents often are essential to help emulsify the thermoset resin and to improve emulsion stability. The presence of organic solvents in a coating composition also improve film coalescence, film flow-out, and substrate wetting.
Waterborne phenoxy resins having a low VOC have been disclosed. These waterborne phenoxy resins are high molecular weight thermoplastic resins that are difficult to process and are too expensive for practical commercial use. In addition, because these phenoxy resins are thermoplastic resins, cured coatings derived therefrom are not resistant to organic solvents, although the cured coatings often provide sufficient barrier properties to water-based compositions for use as can coatings.
Investigators, therefore, have sought a waterborne coating composition for the interior of food and beverage containers that retains the advantageous properties of adhesion, flexibility, chemical resistance, and corrosion inhibition, and that is economical and does not adversely affect the taste or other esthetic properties of sensitive food and beverages packaged in the container. Investigators especially have sought a waterborne coating composition that demonstrates these advantageous properties and also reduces the environmental and toxicological concerns associated with organic solvents.
Investigators prefer a thermosetting coating composition because such compositions are easier to handle, require a lower VOC, and provide better chemical resistance than thermoplastic coating compositions. A thermosetting coating composition, however, typically requires the presence of a crosslinking agent in order to provide a cured coating having a sufficient molecular weight. Generally, the crosslinking agent is a phenolic resin, an aminoplast or a similar resin. Therefore, investigators have sought a low VOC coating composition for food and beverage containers (1) that meets increasingly strict environmental regulations, i.e., has a low VOC and is free of a formaldehyde-containing crosslinking agent, and (2) has corrosion inhibition properties at least equal to existing organic solvent-based or high VOC coating compositions. Such a waterborne coating composition would satisfy a long felt need in the art.
A present waterborne coating composition comprises an epoxy resin, a water-dispersible acrylic resin, an optional solid filler, and a fugitive base. A present waterborne coating composition is self-crosslinkable, and, therefore, is free of a crosslinking agent, like a phenolic resin or an aminoplast. Accordingly, the waterborne coating compositions can be used as a can coating composition for taste-sensitive products, like beer. A present waterborne coating composition contains 450 g or less VOC/kg NVM, yet demonstrates excellent storage stability, composition flow out, substrate wetting, and cured coating properties, such as adhesion, hardness and flexibility.
A waterborne coating composition of the present invention has a very low VOC, and substantially overcomes the environmental and toxicological problems associated with an organic solvent-based coating composition or a high VOC coating composition (i.e., a composition containing greater than 450 g VOC/kg NVM). A present waterborne coating composition also is free of a crosslinking agent, like a phenolic resin or an aminoplast, thereby eliminating the environmental and toxicological concerns associated with formaldehyde, and eliminating an ingredient that can adversely affect the taste of a food or beverage that contacts the cured coating composition.
Prior investigators have studied waterborne epoxy resin-based compositions for application to metal substrates. Many of these investigators sought epoxy resin-based aqueous compositions that provide a sufficiently flexible cured coating such that the coated metal substrate can be deformed without destroying film continuity. Often, conventional epoxy resins provide a rigid cured film thereby making it difficult to impossible to coat the metal substrate prior to deforming, i.e., shaping, the metal substrate into a metal article, like a metal can. Coating a metal substrate prior to shaping the metal substrate is the present standard industrial practice.
For example, Johnson et al. U.S. Pat. No. 4,954,553 discloses an aqueous coating composition comprising a carboxyl-bearing phenoxy resin and a resin that is soft in comparison to the phenoxy resin, like a polyester. The carboxyl-bearing phenoxy resin is prepared by grafting ethylenically unsaturated monomers to the phenoxy resin. The coating composition provides flexible cured coatings. Fan U.S. Pat. Nos. 4,355,122 and 4,374,875 disclose a waterborne phenoxy composition wherein an ethylenically unsaturated monomer including a carboxyl group is grafted onto a phenoxy resin by standard free radical polymerization techniques, then the carboxyl groups are neutralized by a base.
Chu et al. U.S. Pat. No. 4,446,258 discloses an aqueous coating composition comprising: (1) the neutralized reaction product of an epoxy resin with a preformed addition polymer containing carboxyl groups, and (2) an acrylic or vinyl polymer, which is prepared either in situ or added to the composition, and which is different from the preformed addition polymer. Evans et al. U.S. Pat. Nos. 4,212,781 and 4,308,185 disclose grafting an acrylic monomer or monomer blend to an epoxy resin to provide a polymeric blend including unreacted epoxy resin, an acrylic resin and a graft polymer of the acrylic resin and epoxy resin. Steinmetz U.S. Pat. No. 4,302,373 discloses a waterborne coating composition consisting essentially of the neutralized reaction product of a modified polyepoxide (e.g., an ester or ether) and a carboxyl-functional polymer.
Patel U.S. Pat. No. 4,963,602 discloses aqueous coating compositions including an epoxy resin, an acrylic resin, a phenoxy resin, a novolac resin, and a resol resin. Wu U.S. Pat. Nos. 3,943,187 and 3,997,694 disclose coating compositions consisting essentially of a blend of an acrylic polymer having hard and soft segments and an epoxy resin. Wu U.S. Pat. No. 4,021,396 discloses an aqueous coating composition containing an acrylic polymer having at least two reactive sites per chain and an epoxy resin having an epoxy group in combination with at least one hydroxyl or epoxy group. Salensky U.S. Pat. No. 4,638,038 discloses modified phenoxy resins wherein anhydrides or polycarboxylic acids are grafted onto a phenoxy resin. Morinaga et al. U.S. Pat. No. 5,010,132 discloses a coating composition for a metal can comprising: (1) fine particles of a polyester resin including terephthalic acid and isophthalic acid, and (2) a surfactant.
Other patents disclosing coating compositions including an epoxy resin and an acrylic resin include Matthews et al. U.S. Pat. No. 4,234,439; Parekh et al. U.S. Pat. No. 5,387,625; McCarty U.S. Pat. No. 4,444,923; Ting et al. U.S. Pat. No. 4,480,058; Brown et al. U.S. Pat. No. 4,585,813; and Vasta U.S. Pat. No. 3,338,860.
Publications disclosing a water-based coating compositions including an epoxy resin and an acrylic resin are:
J. T. K. Woo et al., "Synthesis and Characterization of Water-Reducible Graft Epoxy Copolymers," J. Coat. Tech., 54 (1982), pp. 41-55; and
R. N. Johnson et al., "Water-Borne Phenoxy Resins Low VOC Coatings with Excellent Toughness, Flexibility and Adhesion," presented at the Water-Borne and Higher-Solid Coatings Symposium, Feb. 3-5, 1988 in New Orleans, La.
Although the above-identified patents and publications disclose coating compositions for the interior of a food or beverage container, the patents and publications do not disclose a self-crosslinkable, low VOC waterborne coating composition that is free of a phenolic or an aminoplast crosslinking agent, and which, after curing, demonstrates: (1) excellent flexibility; (2) excellent adhesion; (3) excellent chemical resistance and corrosion inhibition; and (4) reduced environmental and toxicological concerns.
As an added advantage, it is envisioned that a present waterborne coating composition can be used both on can ends and on can bodies, thereby obviating the use of two different coating compositions by container manufacturers. Furthermore, a present waterborne coating composition exhibits sufficient clarity, hardness, and mar resistance after curing for use as a coating on the exterior of a metal container. Accordingly, because of improved chemical and physical properties, and because of the wide range of cure temperatures, a waterborne coating composition of the present invention has a more universal range of applications, such as for the interior coating of a metal container for food or beverage products or for the exterior coating of a metal container; overcomes the environmental and toxicological concerns associated with a solvent-based or a high VOC coating composition; and overcomes disadvantages, such as adversely affecting the taste of a beverage, presented by prior epoxy-based coatings that contained a phenolic resin or an aminoplast crosslinking agent.